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So Much for So Little: The Modern Soil Treatment Unit

Robert L. Siegrist

There is a need both in the United States and globally for solutions to water and wastewater infrastructure issues that are effective in protecting public health and preserving water quality while also being acceptable, affordable, and sustainable. Onsite and decentralized systems have the potential to achieve these goals in rural areas, peri-urban developments, and urban centers in small and large cities. Moreover, they can improve water-use efficiency, conserve energy, promote green spaces, and help restore water resources. Importantly, onsite and decentralized systems have the potential to yield enormous benefits across the developing world, where estimates are that 1.2 billion people lack clean water supply and 2.5 billion people lack adequate sanitation, resulting in 3.4 million deaths annually from waterborne disease.

A growing array of approaches, devices, and technologies has evolved that have enlarged the “toolbox” of onsite and decentralized systems. These include point-of-use water purification, point-of-generation wastestream separation, anaerobic and aerobic bioreactors, packed-bed filters, constructed wetlands, and soil–aquifer treatment systems. In contrast to years ago, advancements have been made in the science and engineering of these and related unit operations and systems such that rational procedures, including analytical and numerical models, now can enable application-specific engineering to achieve performance goals. Education and training, along with the adoption of effective management programs, have helped ensure achievement of performance goals through proper system selection, design, and operation. Exciting advances in monitoring and sensor technology are enabling remote process control and verification of performance.

When considering the universe of existing onsite and decentralized systems and those that are being implemented every day, the use of “soil” in the treatment system is ubiquitous. Onsite wastewater systems historically relied on soil for disposal of human waste and wastewaters. For much of the 20th century, soil-based waste-disposal systems, such as privies, cesspools, seepage pits, and leachfields, were viewed as methods for keeping human wastes away from people and pollutants out of surface waters. As disposal systems, they were not explicitly designed to achieve a desired treatment performance. As a result, the performance of such disposal systems implemented decades ago can be relatively poor compared to that achieved in modern treatment systems.

The vast majority of treatment systems being implemented today in onsite and decentralized applications across the United States include a unit operation involving soil to achieve tertiary treatment with natural disinfection. Similar to a confined treatment unit (such as a septic tank, packed-bed filter, or membrane bioreactor), an unconfined soil profile can be conceptualized as a wastewater treatment unit operation that is designed to

hydraulically process and purify the effluent within the soil profile to the extent needed to protect public health and water quality;

provide a long service life with low operation and maintenance requirements;

enable resource recovery and reuse; and

be affordable.

Using the terminology soil treatment unit (STU) reflects this conceptualization. The types of systems captured within this contemporary and forward-looking concept include various combinations of confined-unit treatment operations coupled with subsurface infiltration trenches or shallow drip-dispersal networks.

Conceptually, the operating function of an STU is simple; however, with respect to treatment, the underlying mechanisms can be quite complex. During continuous use of an STU across years of operation, the native soil’s capacity to process wastewater effluent can decline from the capacity for clean water prior to effluent application. The rate and extent of decline have been shown to be dependent on the hydraulic loading rate and the levels of total suspended solids and total biochemical oxygen demand in the effluent applied. After some period of operation, an STU can experience a sufficient decline in hydraulic capacity such that intermittent or continuous ponding of the infiltrative surface ensues. However, the time to development of ponding or the sustained occurrence of ponding does not necessarily correlate with long-term hydraulic or treatment performance. Under some conditions, such as when higher daily loading rates occur compared to design assumptions, or after an extended period of continuous use (for example, 20 years or more), hydraulic capacity can decline to a point where rehabilitation may be required. This can be accomplished by operational adjustments, such as resting the previously operated STU or improving the effluent quality applied to it. Biomechanical rehabilitation methods are being investigated that include air injection and robotic cleaning tools.

For tertiary treatment and natural disinfection, unsaturated flow in the soil profile can be critical. This flow regime facilitates contact between wastewater constituents and the soil grain surfaces and their associated biofilms, and provides for a relatively long contact period for treatment processes to occur. Unsaturated flow conditions can be achieved by limiting the design hydraulic loading rate to a small fraction (for example, 1% to 5%) of the soil’s saturated hydraulic conductivity, and that application is achieved by intermittent dosing through pressurized piping networks. Also, over time, effluent infiltration can lead to soil clogging and unsaturated flow conditions irrespective of hydraulic design attributes. Effluent infiltration also can lead to the establishment of a biogeochemically altered zone of soil media at and immediately below the infiltrative surface that can provide more rapid and extensive treatment of the constituents in the applied effluent.

Tertiary treatment can be accomplished in an STU by a combination of naturally occurring physical–chemical and biological processes. Removal of biochemical oxygen demand can occur by biodegradation in biofilms that grow on soil grains and within soil organic matter. Removal of emerging organic compounds of concern, including endocrine-disrupting compounds, has been shown to occur through biodegradation and sorption processes. Suspended solids can be removed by physical filtration and absorption, followed by biodegradation of the organic fraction. Reduced forms of nitrogen can be biologically oxidized, and some total nitrogen can be removed by denitrification. Phosphorus removal varies widely, depending on soil mineralogy and its phosphorus sorption properties. Trace metals can be removed by adsorption and ion exchange. Removal of many pollutants of concern can be enhanced with STUs placed shallowly in the soil profile where plants can aid in nutrient uptake and pollutant removal reactions.

Natural disinfection in the context of an STU is accomplished passively through various natural processes without the active use of chemicals or radiation. Larger pathogens, such as parasites and bacteria, can be filtered out in the soil and die off, while viruses can attach to grain surfaces and be inactivated there or at air–water interfaces that occur during film flow. Effluent infiltration in narrow trenches or drip networks placed shallowly in the soil profile, as well as the development of a biozone at the soil infiltrative surface, can enhance overall pathogen removal and inactivation processes.

Depending on the environmental setting and the scale of the application, the specific performance requirements for an STU can vary. This is in large part due to the role that the receiving environment can play in assimilating the reclaimed water and achieving further polishing treatment of the STU “end-of-pipe” effluent equivalent as reclaimed water moves through the subsurface.

As the future unfolds for onsite and decentralized systems, modern STUs will continue to be widely used as a robust and reliable unit operation that can help achieve tertiary treatment with natural disinfection, and they can do so in a sustainable fashion.

Robert L. Siegrist is director and professor of environmental science and engineering at the Colorado School of Mines (Golden) and director of the school’s Small Flows Program